In a communication system, a preamble or synchronization pattern is used to define the start of a transmit packet. A transmit packet consists of a preamble/synchronization sequence followed by a data protocol sequence transmission. In shared access applications, the transmit carrier is switched on and off for the transmit packet. A number of techniques are currently employed to detect the synchronization pattern.
Many systems rely upon strong signal to noise ratios; some use increased transmit power during the synchronization period to accomplish the same effect. Received signal bit detection is performed and the resulting bit stream is compared to the expected synchronization pattern. When a specified percentage of the input bit stream compares with the expected synchronization pattern, synchronization is assumed.
An analogous effect occurs in a spread spectrum communication receiver which employs a surface acoustic wave (SAW) device. The down converted receive signal is impressed on the SAW device; the SAW device performs an analog comparison of the input signal with an expected bit (chip) sequence; and the SAW device output reflects the instantaneous comparison (or correlation) of the input signal and the expected chip sequence (or symbol). When the input signal is coherent with the expected chip sequence, the SAW device output peaks to indicate that the expected symbol has been received.
In a spread spectrum communication system, the preamble/synchronization pattern may be defined as a specified sequence of symbols. In a rate 1/2 coded system, two symbols are used to represent the two values for each data bit. When a specified percentage of the expected synchronization symbol sequence has been correctly received, synchronization is assumed.
In a poor signal to noise and/or multi-path environment, symbol detection and synchronization become more difficult. Increasing the length of the symbol chip sequence could improve symbol detection and therefore synchronization detection reliability; however, the SAW device (used for symbol detection) has definite limits. The SAW device is constrained by its maximum physical length, manufacturing tolerances, and thermal characteristics.
Employing high speed analog to digital conversion and a digital correlation system to replace the SAW device could enhance symbol detection by allowing much longer symbol chip sequences to be used; however, the resulting system would be considerably more expensive.
Increasing synchronization detect reliability by increasing the number of symbols in the synchronization pattern also has its limits, especially when symbol detection reliability is poor. A long preamble/synchronization symbol pattern may also be detrimental to high transaction volume applications.